The circadian rhythms of core temperature and the sleep/waking cycle are both well-described, but to date their interrelationship has been studied primarily by two means: observations under free running conditions, and after manipulations of the sleep schedule. The present work represents a novel approach: observation of sleep and waking when the core temperature rhythm is held constant. This is accomplished by a "space suit" and computer-driven cooling and heating system developed by the investigators. Using this apparatus, the rhythm of core temperature is abolished, and measures of sleep and subjective experience and motor performance are measured.